It's well documented that weight-bearing exercise strengthens bones and helps ward off osteoporosis. Many postures in yoga require that you lift your own weight. And some poses like Downward- and Upward-Facing Dog, help strengthen the arm bones, which are particularly vulnerable to osteoporosis’ fractures. In an unpublished study conducted at CaliforniaStateUniversity, Los Angeles, yoga practice increased bone density in the vertebrae. Yoga's ability to lower levels of the stress hormone cortisol may help keep calcium in the bones.